The present invention relates to the field of CMOS image sensors and, more particularly, to reducing dark current and hot pixels in CMOS image sensors.
Image sensors, including complimentary metal oxide semiconductor (CMOS) image sensors and charge-coupled devices (CCD), may be used in many different digital imaging applications to capture scenes. An image sensor may include an array of pixels. Each pixel in the array may include at least a photosensitive element for outputting a signal having a magnitude proportional to the intensity of incident light contacting the photosensitive element. When exposed to incident light to capture a scene, each pixel in the array may output a signal having a magnitude corresponding to an intensity of light at one point in the scene. The signals output from each photosensitive element may be processed to form an image representing the captured scene.
As pixels are made smaller, pixel elements may be located closer together in the pixel, resulting in increased risk of cross-talk between adjacent pixels. Further, the supply voltage node must be located close to the photodiode. Shallow trench isolation (STI) regions, which may be dielectric-filled trenches formed in the substrate of the image sensor, may be used to isolate pixels and pixel elements from each other.
While helping to electrically isolate pixels and pixel elements, the STI regions may also create problems in the operation of the pixel cell. For example, STI boundaries may have a higher defect density than the substrate, creating a higher density of “trap sites” along the STI boundaries as compared to the silicon/gate oxide interface or silicon surface that can “trap” electrons or holes. Trap sites may result from defects along the silicon dioxide/silicon interface between the STI boundaries and the silicon substrate. For example, dangling bonds or broken bonds along the silicon dioxide/silicon interface may trap electrons or holes.
Trapped electrons or holes may generate a proportional current at the trap site. The current generation from trap sites inside or near the photosensor may contribute to dark current (i.e., electrical current in the photosensor in the absence of light) in CMOS image sensors since a constant charge may be leaking in the photodiode. Because the readout circuitry of the image sensor may not distinguish between sources of charge in the photosensitive element, dark current may be added to the magnitude of the signal output from the pixel, thus making the pixel appear brighter in the produced image than that point actually appeared in the scene. Such a pixel may be referred to as a hot pixel.
In conventional image sensors, a supply voltage may be applied to the photodiode and the floating diffusion during reset to deplete the photodiode of charge, returning the photodiode to its pinned voltage, and to reset the floating diffusion region. It may be desirable to maintain the supply voltage at a high voltage level (e.g., 2.8 v for mobile applications and 3.3 v for digital camera applications) to reset the floating diffusion to a high voltage and to fully deplete the photodiode. The high supply voltage applied to the supply voltage node may deplete the active area connected to the supply voltage node and may also deplete the photodiode to its pinned voltage (e.g., 1.5 V). When this occurs, a field may be generated between the supply voltage node and the photodiode, which may pull the photodiode depletion region close to the STI edge. Accordingly, dark current may increase at photodiode and STI interface.